For a load control circuit controlling the operation of a load by turning on/off a driving signal therefore, it is necessary to take measures so that it is prevented that the load control circuit is damaged and that a load connected therewith effects no erroneous operation, which can give rise to a dangerous state, when an error is produced in a circuit connecting the load with the load control circuit.
For example, in a load driving control circuit for driving and controlling auxiliary apparatuses such as an air conditioner mounted in a vehicle, etc., when an error is produced in the load driving control circuit itself, a load (a relay, etc.) connected therewith effects an erroneous operation and as the result not only a desired operation is not achieved, but also destruction of elements constituting the control circuit and damage of the battery can be caused by an excessive current.
FIG. 4 is a circuit diagram illustrating an example of this kind of prior art load control circuits, in which Trl is a control transistor; Tr2 is a drive transistor; R1, R2, R4, R5, R6 and R7 are resistors; C is a capacitor; D1 to D3 are diodes; ZD is a Zener diode; RL is a load; DL is an output line; SC is a control signal; SD is a self diagnosing signal; and V1 is a power supply (+12V).
In the figure, when no control signal SC is inputted in the state where the power supply voltage +V is applied (low level: off state), the control transistor Trl is turned-off. Consequently the drive transistor Tr2 is in the off state and no driving current is supplied to the load RL. When the control signal SC is inputted (high level: on state), the control transistor Trl is turned-on and thus the drive transistor Tr2 is also turned on. By the fact that the drive transistor Tr2 is turned on, driving current is supplied from the power supply +V to the load RL through the output line DL. At this time, the self diagnosing signal SD is obtained by driving the potential +V by resistors, i.e. the resistors R2, R1, R8 and the load RL.
In this case, when the control signal SC is at the lower level (non-driven state) and the load RL is connected normally, the potential at the point G is pulled down by the resultant resistance of the resistors R1, R2 and the resistance of the load RL. Therefore it is approximately at 0 V and the self diagnosing signal at the output terminal for the self diagnosing signal is also approximately 0 V. Further, also in the open contact state, where the load RL is disconnected from the output line DL, since the potential at the point G is pulled down by the resistors R1 and R2, it is approximately 0 V and the self diagnosing signal SD is also approximately 0 V.
On the other hand, when the control signal SC is applied thereto (high level: driven state) in the GND short state, where the potential of the drive line DL is dropped to that of the ground line (GND), the base potential of the transistor Trl is at the GND level and the transistor is turned off. Consequently the drive transistor Tr2 is turned off so that the destruction of this transistor is avoided. At this time, since the point G is at the GND potential, the self diagnosing signal is also 0 V.
In this way, it is possible to distinguish the GND short error, where the potential of the drive line for the load RL is dropped to that of the GND line, the open contact error, where the drive line is opened, and the normal state by monitoring the level of the control signal, high or low, and the potential of the self diagnosing signal SD.
By the prior art technique, it is not possible to distinguish the GND short error of the drive line DL in the non-driven state (SC is at the low level), where no control signal is applied thereto and the open contact error of the drive line stated above from each other and when the control signal is applied thereto in that state, the drive control circuit including the drive transistor is damaged at the GND short error. Further, it is necessary to detect a +V power supply short error, where the drive line DL is short-circuited with the +V power supply. In particular, this +V short error causes the damage of the driving control circuit and at the same time it gives rise to an extremely dangerous situation, where the load is driven when the control signal is at the low level.